<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>画圆</title>
  <style>
    .demo{
      width: 250px;
      height: 250px;
      border: 8px solid green;
      border-radius: 50%;
    }
  </style>
</head>
<body>
    <div class="demo"></div>
</body>
</html>